In line with an increase in technology development of and demand for mobile devices, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which have high energy density and operating voltage, a long cycle lifespan, and a low self-discharge rate are commercially available and widely used.
A lithium secondary battery generally includes a positive electrode comprising a positive electrode active material, a negative electrode comprising a negative electrode active material, a separator, and an electrolyte, and is a secondary battery in which charging and discharging are performed by intercalation/deintercalation of lithium ions. Lithium secondary batteries have high energy density and high electromotive force, and can exhibit high capacity, and thus are applied to a variety of fields.
In lithium secondary batteries, a positive electrode active material constituting a positive electrode may be a metal oxide such as LiCoO2, LiMnO2, LiMn2O4, or LiCrO2, and a negative electrode active material constituting a negative electrode may be a carbon-based material such as metal lithium, graphite, activated carbon, or the like, or a material such as silicon oxide (SiOx) or the like.
Among the above negative electrode active materials, metal lithium has high energy density, high voltage, a good charge rate, and a low self-discharge rate, and thus may be usefully used as an active material. Metal lithium used in an electrode of a lithium secondary battery should have a smooth surface, which enables charging and discharging to be performed at a uniform current density on all surfaces. However, existing metal lithium processing processes cannot completely smoothen metal lithium, and, when metal lithium has poor smoothness, charging and discharging occur at a non-uniform current density on a surface of the metal lithium, and thus, as cycles of charging and discharging proceed, lithium atoms are grown on the surface of metal lithium, resulting in damage to a separator, and, accordingly, the formation of lithium dendrites occurs, which breaks down a battery.
Therefore, there is a need to further increase the smoothness of metal lithium so that charging and discharging are performed at a uniform current on a surface of the metal lithium in order for the metal lithium to be used in an electrode of a lithium secondary battery, and, when the smoothness of metal lithium is increased, the formation of lithium dendrites decreases and, accordingly, cycle behavior of a lithium secondary battery may be enhanced and the safety thereof may also be increased.